Marriage defines a distinct relationship.Legalize gay marriage, this IS supposed to be a free country. How is it free if people can't marry who they love? If you don't like gay marriage, then don't get married to the same sex. Simple enough!
All that said I am a straight mother of 3 and I believe in God.
You described marriage in your identity;
In every culture, in all of human history, marriage has been the union of diverse genders who bear the fruit of humanity.
It is the design of evolution and God.
Redundant genders united are sterile. They may be gay unions, but they are not marriage.